Molybdenum disilicide alloy matrix composite

ABSTRACT

Compositions of matter consisting of matrix materials having silicon carbide dispersed throughout them and methods of making the compositions. A matrix material is an alloy of an intermetallic compound, molybdenum disilicide, and at least one secondary component which is a refractory silicide. The silicon carbide dispersant may be in the form of VLS whiskers, VS whiskers, or submicron powder or a mixture of these forms.

This invention is the result of a contract with the Department of Energy(Contract No. W-7405-ENG-36).

This is a division of application Ser. No. 07/462,256 filed 01/09/90,now U.S. Pat. No. 4,970,179.

BACKGROUND OF THE INVENTION

This invention relates to the art of materials science and, moreparticularly, to nonmetallic materials and powder metallurgy.

Ceramic materials have certain outstanding properties, such as hightemperature strength, corrosion resistance, low density, and low thermalexpansion, which make them attractive materials for high temperatureapplications. However, ceramics differ from metals in one very importantaspect; they do not show any yield upon loading. The lack of astress-relieving characteristic, which gives ceramics their brittlenature and low tolerance for flaws, is a major drawback to using them inhigh-temperature structural applications.

There is a class of materials which offers the advantages of a ceramicand certain of the beneficial mechanical characteristics of a metal.These materials are intermetallics, which at high temperature have theexcellent properties of a ceramic, but mechanically behave more like ametal, since they show yielding and stress-relieving characteristics.

Molybdenum disilicide (MoSi₂) is an intermetallic compound which haspotential for structural use in oxidizing environments above 1200° C. Ithas a melting point of 2030° C. and its oxidation resistance at hightemperature is very good. Mechanically, MoSi₂ behaves as a metal at hightemperatures; it undergoes a brittle-to-ductile transition atapproximately 1000° C. Thus, MoSi₂ has a stress-relieving characteristicat high temperatures. The major problems impeding the use of MoSi₂ as ahigh temperature structural material with potential use temperatures inthe range of 1200°-1800° C. are its relatively low strength at hightemperatures and its brittleness, which may be referred to as lack offracture toughness, at low temperatures. Fracture toughness may bedefined as resistance to fracture. At low temperatures, strength islimited by brittle fracture, while at high temperatures, it is limitedby plastic deformation or creep. For this material to be a viablestructural material at high temperatures, both its elevated temperaturestrength and its room temperature fracture toughness must be improved.The present invention addresses the problem of high temperaturestrength, though improvement in low temperature strength and fracturetoughness may also be realized.

Silicon carbide whiskers made by a vapor-liquid-solid (VLS) process havebeen used to reinforce MoSi₂ by means of dispersion strengtheningmechanisms. This resulted in improved ambient temperature fracturetoughness and a near doubling of strength at 1200° C. compared to roomtemperature strength. The use of silicon carbide whiskers made by avapor-solid (VS) process as a reinforcing material provided animprovement over VLS whiskers at high temperatures, but furtherimprovement in strength at high temperatures is needed. This improvementmay be attained by replacing a portion of the MoSi₂ matrix with one ormore refractory metal silicides. The refractory silicide will provide asolid solution strengthening effect or precipitation strengtheningeffect. Also, there are advantages in using SiC in powder form as thereinforcing material.

SUMMARY OF THE INVENTION

This invention is compositions of matter, each consisting of a matrixmaterial having silicon carbide dispersed throughout it, and methods ofmaking the compositions. A matrix material is an alloy of anintermetallic compound, molybdenum disilicide, and at least onesecondary component which is a refractory silicide chosen from a groupconsisting of

tungsten disilicide (WSi₂)

niobium disilicide (NbSi₂)

tantalum disilicide (TaSi₂)

molybdenum trisilicide (Mo₅ Si₃)

tungsten trisilicide (W₅ Si₃)

niobium trisilicide (Nb₅ Si₃)

tantalum trisilicide (Ta₅ Si₃)

titanium trisilicide (Ti₅ Si₃)

titanium disilicide (TiSi₂)

chromium disilicide (CrSi₂)

zirconium disilicide (ZrSi₂)

yttrium disilicide (YSi₂) and

vanadium disilicide (VSi₂).

A matrix alloy may contain more than one of the secondary components.The silicon carbide dispersant may be in the form of VLS whiskers, VSwhiskers, or submicron powder or a mixture of these forms. A matrixalloy has a composition of from about 50 to about 90 mole percent MoSi₂with the balance being one or more of the secondary component refractorysilicides. An inventive composition contains from about 70 to about 90volume percent of the matrix alloy with the balance being siliconcarbide.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 shows yield stress versus temperature for inventive compositionsand other materials.

FIG. 2 shows the ratio of ultimate load to yield load versus temperaturefor inventive compositions.

FIG. 3 shows the magnitude of the displacement at ultimate load versustemperature for inventive compositions.

DETAILED DESCRIPTION OF THE INVENTION

Table I gives certain properties of MoSi₂ and of preferred refractorysilicides which are alloyed with MoSi₂ to form the matrix materials ofthe present invention. Additional refractory silicides which may be usedas secondary components of the matrix alloy are titanium disilicide,chromium disilicide, zirconium disilicide, yttrium disilicide, andvanadium disilicide. A matrix material ranges in composition from about50 to about 90 mole percent MoSi₂ with the balance of from about 10 toabout 50 mole percent being one or more of the refractory silicides. Acomposition of this invention contains from about 70 to about 90 volumepercent of the matrix alloy with the balance of from about 10 to about30 volume percent being silicon carbide (SiC). The SiC used in thisinvention may be in the form of VS whiskers, VLS whiskers, or submicronpowder or a mixture of two or three of these forms.

There are numerous silicides which might be used as secondary componentsof the matrix alloys. Preferred alloying silicides should have a meltingpoint near or higher than that of MoSi₂, good oxidation resistance, andthermodynamic stability with SiC. The preferred refractory silicidesused in this invention were chosen by reference to their density,melting point, strength, oxidation resistance, and compatibility withMoSi₂. For example, NbSi₂ has the desirable characteristics of a lowerdensity than MoSi₂ combined with a reasonably high melting point. TaSi₂has a high density, which is a disadvantage, but its higher meltingpoint than MoSi₂ makes it useful in the present invention. Othersilicides which are not in the preferred group but might be desirablesecondary matrix components in certain applications are the disilicidesof titanium, chromium, zirconium, yttrium, and vanadium; thesedisilicides were not used in the inventive compositions which have beenmade to date. All of the secondary matrix component disilicides areexpected to form solid solutions with MoSi₂.

                  TABLE I                                                         ______________________________________                                        Matrix Materials                                                                      Melting       Crystal   Density                                       Silicide                                                                              Point, C.     Structure g/cm.sup.3                                    ______________________________________                                        MoSi.sub.2                                                                            2030          Tetragonal                                                                              6.24                                          WSi.sub.2                                                                             2160          Tetragonal                                                                              9.86                                          NbSi.sub.2                                                                            1930          Hexagonal 5.66                                          TaSi.sub.2                                                                            2200          Hexagonal 9.1                                           Mo.sub.5 Si.sub.3                                                                     2160          Tetragonal                                                                              8.24                                          W.sub.5 Si.sub.3                                                                      2370          Tetragonal                                                                              14.5                                          Nb.sub.5 Si.sub.3                                                                     2480          Tetragonal                                                                              7.16                                          Ta.sub.5 Si.sub.3                                                                     2500          Tetragonal                                                                              13.4                                          Ti.sub.5 Si.sub.3                                                                     2130          Hexagonal 4.32                                          ______________________________________                                    

The refractory trisilicides were chosen because their high meltingpoints are expected to cause an improvement in high temperature strengthand creep resistance. It is expected that they will provideprecipitation strengthened microstructures such as that of Mo₅ Si₃dispersed in MoSi₂.

Samples of compositions of this invention were made using alloys of 50mole percent MoSi₂ and 50 mole percent of each of the eight alloyingsilicides of Table I. These matrix materials were mixed with 30 volumepercent submicron SiC powder to form the inventive compositions. TableII gives values of percent of theoretical density for these samples,which were made as described below. Values are presented for matrixalloys having 50 mole percent of each component without SiCreinforcement and for inventive compositions having 50/50 matrices and30 volume percent of submicron SiC powder.

                  TABLE II                                                        ______________________________________                                        Percent of Theoretical Density                                                Hot Pressed  Density Percent                                                                           Density Percent                                      Compound     Without SiC With SiC Powder                                      ______________________________________                                        MoSi.sub.2   94.5        93.3                                                 MoSi.sub.2 /WSi.sub.2                                                                      92.8        95.0                                                 MoSi.sub.2 /NbSi.sub.2                                                                     93.4        95.1                                                 MoSi.sub.2 /TaSi.sub.2                                                                     97.5        98.5                                                 MoSi.sub.2 /Mo.sub.5 Si.sub.3                                                              96.0        91.4                                                 MoSi.sub.2 /W.sub.5 Si.sub.3                                                               80.3        71.2                                                 MoSi.sub.2 /Nb.sub.5 Si.sub.3                                                              97.1        99.4                                                 MoSi.sub.2 /Ta.sub.5 Si.sub.3                                                              92.1        100.0                                                MoSi.sub.2 /Ti.sub.5 Si.sub.3                                                              99.9        100.0                                                ______________________________________                                    

Strength data has been collected only on compositions having a matrixalloy of 50 mole percent MoSi₂ and 50 mole percent WSi₂ and containing80 volume percent matrix and 20 volume percent SiC. WSi₂ was selectedfor use in initial strength testing primarily because it has high yieldstrength and also has the same crystal structure as MoSi₂ and nearly thesame lattice parameters. Thus, WSi₂ can be expected to readily formsolid solution alloys with MoSi₂. Additionally, the oxidation resistanceof WSi₂ is reasonable, although not as good as that of MoSi₂. The 50/50matrix alloy was selected for the initial testing so that the maximumalloying effect would be easily seen. It is expected that the amount ofthe secondary component used in the matrix alloy will depend on theapplication. For example, WSi₂ is more dense than MoSi₂, so that forapplications where weight is important, such as aerospace applications,the minimum amount of WSi₂ which provides the needed properties will beused. MoSi.sub. 2 has been selected as the primary matrix alloycomponent on the basis of its desirable properties and to use less than50 mole percent of MoSi₂ would cause loss of the benefit of thoseproperties. For example, the oxidation resistance of MoSi₂ is superiorto that of any of the secondary matrix components.

A minimum of about 10% by volume of the dispersed material is requiredbecause less would not provide the significant strengthening effectwhich is desired. It is difficult to make compositions having more thanabout 30 volume percent whiskers due to problems of blending thewhiskers with the matrix alloy powder and the tendency of the whiskersto retard densification by means of skeletal interlock. When thereinforcing material is in the form of an equiaxed powder, largeramounts than 30 volume percent may be used, but to do so blurs thedistinction between matrix and reinforcing material. The properties ofthe composition should be dictated by the matrix alloy properties andprimarily by the properties of MoSi₂

VLS beta-SiC whiskers made at Los Alamos National Laboratory or VSbeta-SiC whiskers purchased from J. M. Huber Corporation of Borger,Texas and designated Huber XPW2 whiskers were used to make compositionswhich were subjected to strength testing. Those skilled in the art arefamiliar with methods of making both types of whiskers. SiC whiskers areminute, high-purity, single crystal fibers. They have very highstiffness in the longitudinal direction, in which they are grown. Themain difference between the two whisker types used is their size. Inhot-pressed shapes of the inventive compositions, VLS whiskers wereabout 50 to 100 microns long, about 3 to 15 microns in diameter, and hadan aspect ratio ranging from about 10:1 to about 20:1. VS whiskers inhot-pressed shapes were from about 1 to about 5 microns in length, had adiameter of from about 0.1 to about 0.5 micron, and had an aspect ratioof from about 5:1 to about 15:1. A few VS whiskers were 100 to 200microns long in the as-purchased condition, but were broken down to lessthan 5 microns long during processing. Submicron SiC powder designatedas Grade A and having an average particle diameter of 0.5 micron waspurchased from the German Company H. C. Starck. MoSi₂ powder of 99.9%purity was purchased from Alfa Products of Danvers, MA. Of the secondarymatrix alloy components, Alfa supplied the tungsten silicides. Mo₅ Si₃and Nb₅ Si₃ were made by the inventors using methods familiar to thoseskilled in the art and the balance of the secondary components werepurchased from Cerac Inc. of Milwaukee, WI.

SiC whiskers were pretreated before they were used in making theinventive compounds. VLS whiskers were subjected to sedimentation toremove catalyst balls and other extraneous matter. In order to eliminateclumps of whiskers and extraneous matter, a wet processing method wasused for VS whiskers, as follows. Whiskers were dispersed in deionizedwater at a pH of about 9.5. The pH may be adjusted by use of any commonbase, such as sodium hydroxide or ammonium hydroxide. Ammonium hydroxidewas used in this case. Experimentation in which the pH is varied has notbeen accomplished, but it is believed that values on the alkaline sidewill be satisfactory with preferred values being above about 9.5. Theamount of water used was large compared to the quantity of whiskers.About 300 ml of water per 1 g of whiskers was used, which is equivalentto 0.33 wt % of whiskers in water. However, the amount of water used perunit of whiskers is not critical and may vary. Dispersion of thewhiskers may be accomplished by stirring, but in order to increase theyield of usable whiskers, it is believed desirable to provide a highdegree of agitation to the whisker-containing water. In ourexperimentation, high shear homogenation, by means of an appropriatelyconfigured agitator, and sonification were used. After dispersion, thewater-whisker slurry was allowed to stand for about five minutes inorder to allow matter to settle to the bottom of the vessel containingthe slurry. The supernatant, which contained whiskers in suspension, wasrecovered by decantation. The supernatant was allowed to stand for about24 hours and then the liquid was drained from the sediment consisting ofwhiskers, thus completing the pretreatment process. However, ifnecessary, the whiskers can be dried by any convenient method. Theduration of the settling periods may vary widely from the five minutesand 24 hours used; the suspensions must stand for a sufficient period toeffect the necessary separations. Those skilled in the art willappreciate that this wet processing method may be varied or that othermethods may be used to accomplish removing extraneous matter and clumpsfrom batches of whiskers. Such methods include separation by mechanicalmeans such as centrifugation.

All of the inventive compositions were made in the same manner, asillustrated by the following example.

EXAMPLE

MoSi₂ powder and WSi₂ powder were separately screened to obtain powderwhich passed through a 400 mesh screen (opening of approximately 37microns) and the resulting -400 mesh powders were blended in a highspeed mechanical blender in the amounts required to provide 50 molepercent of each.

An aqueous slip suspension containing the matrix alloy powder and SiCpowder in amounts to yield a composition of 20 volume percentreinforcing material was prepared. The solids loading of the slip wasabout 50 weight percent. The amount of solids is not critical, but ispreferably from about 40% to about 65 wt %. Deionized water having a pHadjusted to 9.5 with ammonium hydroxide was used to make the slip. Thesuspension was mechanically stirred and ultrasonified to keep theconstituents from settling before casting was accomplished. The slip wascast into a plaster of paris mold and allowed to set. The green slipcast body was dried and then comminuted to -10 mesh (less than 2 mm)shards to yield a material suitable for hot pressing.

The comminuted material was placed in a Grafoil® lined die andhot-pressed into disks measuring approximately 31.8 mm in diameter by6.35 mm thick. Hot pressing was performed in argon and temperatures weremeasured optically. A load of 30 MPa was applied as the increasingtemperature reached 1200 C. Press movement stopped when the temperaturewas about 1900 C., at which point heating was stopped and a hold periodstarted. Hold time at the peak temperature of about 1900 C. was about 5minutes and then slow cooling was started, though it may be desirable touse a longer hold time of up to about one hour. When the decreasingtemperature reached 1200 C., the load was slowly removed and the sampleallowed to cool to room temperature.

Slip casting to form a green body and treating it by means of a sizereduction process is done mainly to provide a material which is betteradapted for hot pressing than a dry mixture of the components; thoughslip casting and grinding does also enhance dispersion. However, thoseskilled in the art are familiar with other methods of preparing materialfor hot pressing which are applicable to the compositions of thisinvention. Pressureless sintering (applying heat only) of a dry blend ofmaterials may also be used to make the inventive compositions. Thisprocess is expected to be especially effective when the secondary matrixalloy is a disilicide of titanium, chromium, vanadium, yttrium, orzirconium, as these disilicides have relatively low melting points.

SiC and MoSi₂ are thermodynamically stable chemical species. X-raydiffraction analyses of the SiC-MoSi₂ /WSi₂ composites did not indicatethe presence of any reaction phases, nor was any reaction observed lightoptically or in the scanning election microscope (SEM). This indicatedthat the system SiC-MoSi₂ -WSi₂ is also a thermodynamically stable one,at least up to the hot pressing temperature of 1900 C. SEM energydispersion x-ray analyses showed that significant solid solutionalloying of the MoSi₂ and WSi₂ powders took place upon hot pressing.X-ray diffraction analyses also indicated that a single phase MoSi₂-WSi₂ solid solution was developed as a result of hot pressing.

Elevated temperature four-point bend tests were performed oncompositions of the invention and on specimens of pure MoSi₂ andwhisker-reinforced MoSi₂ for purposes of comparison at temperature of1200 C., 1400 C., and 1500 C. All testing was performed in air using anInstron mechanical testing unit, a MoSi₂ element furnace, SiC loadingrams, and a SiC pin-Si₃ N₄ base bend test fixture. Four-point bend testsare a method for determining the strength of a material in a relativelysimple and inexpensive manner. This type of test utilizes compressiveloading, which allows the test to be easily run at high temperatures.Note that strengths of ceramics may vary widely in accordance with thetype of test used to determine strength. The test equipment, methods ofconducting tests, and the equation used to solve for strength values areknown to those skilled in the art.

Test members in the shape of rectangular bars having the dimensions2.5×5.1×25.4 mm long were diamond machined from the hot pressed disks.Preliminary investigation indicated that these materials could also beelectrodischarge machined. Two load points on a 5.1 mm wide face of thetest member were 9.5 mm apart and the other two load points on theopposite face were 19.0 mm apart. Each of the tests was duplicatedseveral times and the results reported in the Figures are averages ofseveral tests. The test members were soaked at temperature for about 1/2hour to allow equilibration. The test members were loaded using aconstant strain rate of 0.0508 mm/min.

FIG. 1 presents yield stress versus temperature. Yield stress was takenas the stress which caused a 0.05 mm permanent plastic offset deviation.Yield stresses for all of the composites are significantly greater thanthat of pure MoSi₂. Values for the inventive compositions are, with theexception of one data point, greater than the pure MoSi₂ -VS whiskerscomposite. At 1200 C., the VS SiC whisker-MoSi₂ /WSi₂ matrix compositeexhibited the highest strength, reaching a level of nearly 600 MPa, avalue over 4 times higher than pure MoSi₂. At 1500 C., composite yieldstrengths were 8-10 times higher than that of pure MoSi₂. Strengthvalues as high as 80 MPa were observed in two inventive compositions at1500 C. These composite yield strength values are of significance forpotential engineering applications.

Comparison of the yield stress levels of VS SiC whisker-MoSi₂ matrixcomposites and VS SiC whisker-MoSi₂ /WSi₂ matrix composites shows thatan improvement in yield strength occurred as a result of solid solutionalloying of the matrix. Since the MoSi₂ matrix deforms by dislocationplasticity at elevated temperatures, this result indicates thatdislocation motion is made more difficult due to the presence ofsubstitutional alloying species in the matrix.

Alloy matrix composites reinforced with VS SiC whiskers exhibited thehighest strengths at 1200 C. However, at 1500 C., all types of SiCreinforcements showed similar strength levels. The results for thesubmicron SiC powders are particularly interesting from the viewpoint offuture composite development. SiC powders are more widely available thanSiC whiskers and are significantly less expensive. In addition, it maybe easier to fabricate composites with higher volume fractions ofreinforcement material using equiaxed submicron SiC powder as comparedto elongated SiC whiskers. Also, whiskers are considered to be hazardousto health whereas SiC powder is not.

FIG. 2 shows the ratio of the ultimate load to the yield load versustemperature. Above 1200 C., the inventive compositions behave in thesame manner as metals by continuing to bear load after the yield stressis exceeded rather than failing catastrophically. The ultimate load,which is the load causing the specimen to fail, is presented in FIG. 2rather than the ultimate stress, since the bend tests which wereperformed do not provide reliable data for calculating ultimate stress.Ultimate stress values which are calculated assuming linear elasticbehavior can significantly overestimate the actual ultimate strength,since the stress state changes upon yield in a nonlinear manner. At 1200C., the ratios were 1.0, but the increased with increasing temperature,indicating that strain hardening took place. The VS SiC whiskercomposites exhibited the highest values of ultimate load/yield loadratio.

FIG. 3 provides an indication of elevated temperature ductility. Bendingplastic displacement versus temperature is shown. The MoSi₂ /WSi₂composites were less ductile than the MoSi₂ composite, especially at1200 C. Of the alloy matrix composites, the VS SiC whisker reinforcedmaterial exhibited the highest ductility.

Specimens of MoSi₂ with SiC were hot pressed at 1800 C. rather than 1900C.

The oxidation resistance of the inventive specimens was good incomparison with that of pure MoSi₂. Visual inspection of the specimensafter oxidation exposure in air at 1500 C. for 1-2 hours in connectionwith mechanical testing showed the specimens to be little oxidized.

Since MoSi₂ behaves like a metal at high temperatures, it is believedthat its deformation mechanism is dislocation plasticity, wheredislocations, or flaws in the atomic structure, are caused to move by anapplied stress, thus producing macroscopic deformation. It is believedthat the reinforcing materials pin dislocations, that is, make thedislocations more difficult to move, so that the stress required to movethem is larger; this increases macroscopic strength. The preferredparticle size of SiC powder used in the inventive compositions is fromabout 0.1 to about 1.0 micron. It is believed that use of largeparticles, say 10 micron and larger, will not block dislocation movementas well as smaller particles and that nanosize particles will bedifficult to disperse in the matrix material. Submicron powder isdefined as consisting substantially of powder particles in the sizerange 0.1 to 1.0 micron, though particles up to about 10 microns anddown to about nanosize particles may also be present.

In the range of 300 to 600 C., there exists a region where MoSi₂ can becompletely disintegrated by a oxidation process involving molybdenumtrioxide (MoO₃). The process is highly dependent on the microstructureand occurs only though the pore channels in the MoSi₂. Therefore, theproblem is a function of porosity, and if the porosity can becontrolled, the problem is eliminated. There is no reaction betweenMoSi₂ and SiC, even at temperatures up to the melting point of MoSi₂.

What is claimed is:
 1. A method of making a composition of matterconsisting of a matrix substance having silicon carbide dispersedthroughout it, said matrix substance consisting of molybdenum disilicideand at least one refractory silicide chosen from a group consisting oftungsten disilicide, niobium disilicide, tantalum disilicide, molybdenumtrisilicide, tungsten trisilicide, niobium trisilicide, tantalumtrisilicide, titanium trisilicide, titanium disilicide, chromiumdisilicide, zirconium disilicide, yttrium disilicide, and vanadiumdisilicide, where said matrix substance consists of from about 50 toabout 90 mole percent of molybdenum disilicide, where said siliconcarbide is present in an amount of from about 10 to about 30 volumepercent, and where said silicon carbide is in the form of submicronpowder or VS whiskers or VLS whiskers or a mixture of two or three ofthree of these forms, said method comprising:a. mixing molybdenumdisilicide powder, at least one of said refractory silicides in powderform, and said silicon carbide to form a substantially homogenousmixture; b. applying heat to said mixture until a peak temperature ofabout 1900° C. is reached; c. applying pressure to said mixture andincreasing the pressure on said mixture until a peak pressure of about30 MPa is reached; d. holding said mixture at about said peak pressureand about said peak temperature for a time period of from about 5minutes to about 1 hour to form a coherent shape; e. decreasing thepressure applied to said coherent shape to atmospheric pressure andcooling said coherent shape to ambient temperature; and f. recoveringsaid coherent shape.
 2. The method of claim 1 where said silicon carbideis in whisker form and said whiskers are processed before being mixedwith said matrix substance, said processing being comprised ofseparating the whiskers into a fraction comprised substantially ofnon-agglomerated whiskers and a fraction comprised of agglomeratedwhiskers and extraneous matter, where only said non-agglomeratedwhiskers are mixed with the matrix substance.
 3. The method of claim 2where said separation is accomplished by:a. dispersing said siliconcarbide whiskers in water; b. allowing the resulting suspension to standso that gravity sedimentation takes place; c. separating the resultingsediment from the supernatant, which contains non-agglomerated whiskers;d. allowing the whisker-containing supernatant to stand so that gravitysedimentation takes place; and e. separating the resulting sedimentcomprised of whiskers from the liquid.
 4. The method of claim 1 wheresaid mixture of molybdenum disilicide, at least one of said refactorysilicides, and silicon carbide is subjected to the following stepsbefore said heat and said pressure are applied:a. adding water to saidmixture and mixing to form a suspension; b. casting said suspension in amold; c. drying said casting; and d. treating said dried casting toreduce it to a powder suited for hot-pressing.